The forward and backward azimuths differ from 180 degrees due to which effect (except when A and B lie on the same meridian or at latitude zero)?

• A. Convergence of meridian
• B. Dip of the ellipsoid
• C. Height correction
• D. Geodesic curvature

Answer: (A)

Azimuths are measured with respect to the local meridian (the north–south line) at each point. On the ellipsoid, meridians bend toward the poles, so the north direction at A and the north direction at B are not parallel. Because the forward azimuth (direction from A to B) is defined relative to A’s local meridian and the backward azimuth (direction from B to A) is defined relative to B’s local meridian, the two angles don’t line up as exact opposites except in special cases. This changing reference direction between A and B is called meridian convergence. When A and B lie on the same meridian or when at latitude zero (the equator), the meridians effectively align in a way that the two azimuths differ by exactly 180 degrees. Thus the effect causing the forward and backward azimuths not to sum to 180 degrees in general is the convergence of meridians.